Exercises repetition counters have been frequently used by professional athletes as well as amateurs in sport activities to provide accurate and reliable counting of repetitive actions. Such repetitions counters include pedometers, swim lap counters, weight lifting counters, etc. Traditional pedometers usually utilize a one-dimensional pendulum sensor which requires the user to mount the pedometer on the waist. When the user takes one step, the pendulum sensor increments the step count. However, this kind of pedometer has strict requirement on how it is worn (especially the orientation) on the user and the accuracy is quite limited because the orientation of the pedometer must be strictly kept.
In order to achieve more accurate counting function and less wearing restrictions, exercise repetition counters with more than one sensor have been developed. Examples of such exercise repetition counters are disclosed in U.S. Pat. No. 6,700,499, where a pedometer count the step number in a specified direction selectively from a plurality of sensors. Such specified direction is determined by selecting a reference axis and then the counting is based on the selected reference axis. This type of configuration, however, is not adaptable enough as there are delays in changing to a new reference axis when the orientation of the pedometer is frequently changed in a short period.
Another publication EP 1,813,916 discloses a pedometer containing a three dimensional (3D) accelerometer. The pedometer detects and updates step counts based on an available acceleration signal that is extracted from raw acceleration data with a threshold value. Moreover, the thresholds are updated only based on the amplitude of the waveform as measured by the accelerometer.